Effects of vinyl substitutions on resonance Raman spectra of (bacterio)chlorophylls

Soret resonance and Qy preresonance Raman spectra are reported and compared for a series of (bacterio)chlorophylls. Chlorophyll a, 2-acetylchlorophyll a, bacteriochlorophyll a and 2-vinylbacteriochlorophyll a were studied in the non-protic solvent tetrahydrofuran. These experiments were designed to identify Raman bands corresponding to the stretching mode(s) of the vinyl group at the C-2 position of ring I of chlorophyll a and 2-vinylbacteriochlorophyll a, and to ascertain whether additional bands corresponding to Ca Cm and/or Cb Cb vibrations could be observed in the 1615-1660 cm-1 region. Raman spectra of chlorophyll a and 2-vinylbacteriochlorophyll a exhibit a 1625 cm-1 band, which is absent from the Raman spectra of 2-acetylchlorophyll a and bacteriochlorophyll a. It is assigned to the vC2a C2b mode of the vinyl group. No other band can be definitively assigned to any mode predominantly arising from vinyl motions. The acetyl-containing molecules 2-acetylchlorophyll a and bacteriochlorophyll a give rise to a ca. 1070 cm-1 band, which appears to be related to the presence of the acetyl substituent. The 1615-1660 cm-1 region of the Raman spectra of all four derivatives did not contain any additional band which could be ascribed to modes involving the vCa Cm and/or Cb Cb coordinates